Pacemakers for generating artificial stimulating pulses for the heart, and which may or may not be implanted in the body, are well-known. Pacemakers can be classified into demand and non-demand types. A demand pacemaker only issues an artificial pulse if the heart does not produce its own satisfactory natural beat, whereas a non-demand pacemaker issues artificial stimulating pulses without regard to the presence or absence of a natural beat.
A demand pacemaker normally includes an input amplifier for receiving and amplifying electrical signals from the heart (which signals might result from either a natural beat or an artificial pulse which has just been generated by the pacemaker), a pacemaker control circuitry which receives the amplified signals and which causes a new artificial stimulating pulse to be generated (for transmission to the heart) only if the amplified signals, or lack thereof, show that an artificial stimulating pulse is required by the heart (i.e. on demand), and an output amplifier which receives and amplifies the artificial pulses generated by the control circuitry, for passage to the heart.
Many types of pacemaker control circuitry as described above are available. Some function on an analog basis to produce the accurately-timed artificial stimulating pulses, whereas several recent designs employ digital circuitry.
With demand pacemakers, it is highly desirable to provide protection for the input circuitry from noise. Should a person bearing an implanted demand pacemaker meet an environment of high electrical noise, this noise may be picked up by the input amplifier which may then be incapable of distinguishing the picked-up signals from the signals it normally receives from the heart. In such circumstances, the circuitry is likely to consider the noise signals in the same manner as if it had received signals arising from a natural heart beat--it will operate in its usual demand mode by not issuing artificial stimulating pulses to the heart. Since the heart may not be operating normally, and may indeed require artificial stimulation, these noise signals preventing such stimulation can lead to a dangerous condition for the person concerned.
Noise protection circuits have been incorporated into demand pacemakers in the past. For example, several include filters to attenuate noise signals of particular frequencies (particularly the commercial power mains frequency).